(1) Field of the Invention
This invention relates to a current comparison type differential amplifier. More particularly, it relates to a differential amplifier employing current-mirror circuits which are composed of NPN transistors that are operated in the inverse mode (hereinbelow, simply termed "inverse NPN transistors") or which are composed of integrated injection logic (hereinafter, abbreviated to "I.sup.2 L").
(2) Description of the Prior Art
Differential amplifiers which have heretofore been known are circuits of the voltage comparison type which amplifies the voltage difference between two input signals. FIG. 1A of the accompanying drawings shows a prior-art differential amplifier.
As shown in the figure, the emitters of a pair of transistors Q.sub.1 and Q.sub.2 are connected in common and connected to a constant current source I. Since the constant current source I has a very high impedance, the respective transistors Q.sub.1 and Q.sub.2 effect the emitter-follower operation and their input impedances become very high. If the transistors Q.sub.1 and Q.sub.2 and loads R.sub.1 and R.sub.2 are chosen to be substantially identical respectively and input voltages V.sub.I1 and V.sub.I2 are equal voltages, currents to flow through the respective transistors Q.sub.1 and Q.sub.2 will become equal. If the input voltage V.sub.I1 is greater than the input voltage V.sub.I2, the current of the transistor Q.sub.1 will become greater than that of the transistor Q.sub.2. Since both the transistors have the emitters coupled and connected to the constant current source, the current increment of the transistor Q.sub.1 and the current decrement of the transistor Q.sub.2 become equal, and a voltage increment which is proportional to the current decrement of the transistor Q.sub.2 appears in an output voltage V.sub.out.
In this manner, the prior-art differential amplifier is such that since the input impedances are very high, the input currents are small, the output being provided by comparing the input voltages.
With the prior-art differential amplifier, the respective transistors and resistors need to be fabricated in an isolated manner within an integrated circuit. Usually, also the constant current source in FIG. 1A is an NPN transistor. An example of a layout pattern in the case of the differential amplifier of FIG. 1A within the integrated circuit is shown in FIG. 1B. As illustrated in FIG. 1B, the transistors Q.sub.1 and Q.sub.2 constituting the differential pair, the transistor Q.sub.3 for the constant current source, and the resistors R.sub.1 and R.sub.2 must be formed in island regions 11 respectively enclosed with an isolation region 10. Therefore, the area which the differential amplifier occupies in the integrated circuit becomes large.
In the case where, as an application of the differential amplifier, a photocell is used for the input of the voltage comparison type differential amplifier, the circuit arrangement is as shown in FIG. 1C. Inputs (+) and (-) in FIG. 1C correspond to the inputs V.sub.I1 and V.sub.I2 in FIG. 1A. D.sub.1 designates a diode being the photocell, which provides a current proportional to a quantity of light. D.sub.2 designates a diode for converting into a voltage the current produced by the diode D.sub.1.
When the optical input is feeble, the current of the diode D.sub.1 naturally becomes very small, and it becomes 100 pA--several hundreds pA or so in some cases. Even under such a state, the differential amplifier indicated at A needs to effect a precise amplification in response to the input signal. The diode D.sub.1 is connected in parallel between the input of the differential amplifier and the earth side. Therefore, if the current produced by the diode D.sub.1 flows into the input side of the differential amplifier, precise amplification will be impossible. For this reason, a very great value is required for the impedance of the differential amplifier, and the specification of the input current needs to be several tens pA or less.
In the case where the prior-art differential amplifier is used together with an I.sup.2 L, the signal levels of both the circuits are different, and hence, the output of the differential amplifier needs to be translated to the signal level of the I.sup.2 L (or vice versa). In general, the output signal of the differential amplifier has a higher voltage level than the signal of the I.sup.2 L. For the level translation, a level shift circuit is necessary besides the circuit shown in FIG. 1A, the circuit area increases still more.